A projection photolithography tool is a device that projects an image of a mask pattern onto a wafer by means of a projection objective. In order to form a projection image with relatively high accuracy on the wafer, it is necessary to accurately bring the wafer to a specified exposure position using an automatic focusing and leveling apparatus.
U.S. Pat. No. 4,558,949 describes a detection device for focusing and leveling, as shown in FIG. 1. This detection device includes: an illumination unit 101, a projection slit 102, a first planar reflector 103, a second planar reflector 105, a scanning mirror 106, a detection slit 107 and a photodetector 108. Light emitted from the illumination unit 101 passes through the projection slit 102 and is then reflected by the first planar reflector 103 onto a wafer surface 104, forming thereon a projection spot. The wafer surface 104 reflects the light onto the second planar reflector 105 which, in turn, reflects the light to the scanning mirror 106. The scanning mirror 106 modulates the light signal by periodically oscillating in a simple harmonic motion in order to increase its signal-to-noise ratio (SNR). The light from the scanning mirror 106 passes through the detection slit 107 and is incident on the photodetector 108 which then produces a voltage signal corresponding to the intensity of the received light. Under the effect of the modulation by the scanning mirror 106, the signal output from the photodetector 108 is eventually a periodically varying dynamic voltage signal. Finally, the dynamic voltage signal combined with a feedback square wave from the scanning mirror is analyzed and processed to detect defocus of the wafer surface 104. As a modulation reference for the focusing and leveling system, the scanning mirror needs to operate for a long time and are hence susceptible to the impact of temperature, pressure, humidity and other factors, which may impair its operational stability and thus degrade the wafer surface defocus detection accuracy of the detection device.
FIG. 2 shows demodulation result vs. defocus profiles at ideal and actual amplitude values. The profiles shown in FIG. 2 are obtained from a phase difference detection method. As apparent from the figure, amplitude stability of the scanning mirror has a great impact on the defocus detection. Therefore, this method suffers from certain limitations.
Persons skilled in this art have been looking for a solution for increasing wafer surface defocus measurement accuracy for such a detection device.